X-Ray Computed Tomography (CT) was introduced in the late 1970s as a means for forming three dimensional images of human anatomy. Although its initial spatial resolution was inferior to that of film radiography, it brought a new level of contrast resolution that enabled radiologists to discern previously undetectable low contrast pathology. The apparatus configuration evolved through many generations and is often configured using a rotating X-ray source which is opposed by a detector array rotating in fixed relationship to the source. The x-ray source is often mounted on a C-arm system or conventional gantry. The detector arrays often consist of two dimensional arrays of discrete detectors in conventional CT or in the form of a large area cone beam flat panel detector in C-Arm CT.
Conventional CT is used for a wide range of diagnostic tasks and generally rather scatter free signal detection due to the smaller area of the detector arrays, although these areas are increasing in recent years. C-Arm CT is typically used for interventional procedures where it has been recently possible to obtain 3D Digital Subtraction Angiographic data reconstructions by performing a CT angiogram following the introduction of iodine contrast into the vascular system. CT angiography can also be implemented on conventional CT systems but due to the small detector area, the injected contrast bolus must be followed and the timing of the gantry or table advanced relative to the bolus traversal, which can pose timing problems that result images being obtained during suboptimal opacification.
In the 1980's the concept of spiral CT was introduced. In this mode, rather than obtaining one slice at a time, the table is advanced through the rotating gantry and the x-rays passed through the patient in a helical fashion. Using data interpolation, reconstruction of a series of CT images of sequential planes can be quickly obtained.
One of the limitations of conventional CT from its inception is the fact that a rather uniform distribution of X-rays is sent to all parts of the anatomy regardless of the anatomical thickness. Because of this, dose is more than necessary in thin regions and the X-ray beam parameters are generally chosen to guarantee penetration of the least transmissive regions.
DIGITAL BEAM ATTENUATOR, Mistretta et al. U.S. Pat. No. 4,497,062 (1985) describes a digitally controlled x-ray beam attenuation method and apparatus. A large area phosphor plate detector was used to digitally record a chest radiograph. The information from this radiograph was used to drive a dot matrix printer equipped with a cerium ribbon. The printer produced a Cerium mask that filtered the incident X-ray beam used for a subsequent chest radiography in which an optimal amount of radiation was sent to each point of the patient's chest. Smaller amounts of radiation were sent to the lungs than to the diaphragm and mediastinum. The result was increased detection of lung nodules in the typically underpenetrated regions.
Deasy et al. (U.S. Pat. No. 5,668,371) describes a multi-leaf collimator system for tailoring the depth of proton Bragg peaks in proton radiotherapy applications.
It would be advantageous to overcome the deficiencies described above, specifically those directed at correcting a major deficiency in C-Arm CT in which contrast resolution is inferior to that of conventional CT. For this reason it has been difficult to adequately visualize areas in the brain where bleeding has occurred using the C-Arm equipment typically available in the neuro-interventional suite. Patients typically must be also sent to conventional CT for visualization of subtle soil tissue images.